A nonvolatile magnetic random access memory (MRAM) is an array of memory cells formed on intersections of word lines and sense lines, each memory cell typically having two magnetic layers separated by a conductive or insulating layer. IBM's U.S. Pat. No. 5,343,422 describes an MRAM array wherein the memory cells operate on the basis of giant magnetoresistance (GMR), where each cell is a "spin-valve" cell comprising two thin layers of ferromagnetic material separated by a thin layer of nonmagnetic metallic conducting material, such as copper. IBM's U.S. Pat. No. 5,640,343 describes an MRAM array formed from magnetic tunnel junction (MTJ) memory cells, where each cell comprises two thin layers of ferromagnetic material separated by a thin layer of insulating material, such as alumina. In the GMR type of MRAM array, the magnetization easy-axis of both ferromagnetic layers in each memory cell or storage element is oriented substantially lengthwise of the storage elements and substantially parallel to the direction of an applied sense current. In the MTJ type of MRAM array, the sense current is perpendicular to the plane of the magnetic layers. The magnetic moment or magnetization direction of one of the ferromagnetic layers is fixed in a direction substantially lengthwise of the storage elements, and the magnetization direction of the other layer is free to switch between two digital states in which the magnetization is substantially parallel or substantially antiparallel to the magnetization direction in the one layer.
To fabricate a large and reliably accessed MRAM array containing thousands or millions of cells on a single chip, uniformity and predictability of the magnetic response characteristic of each cell is of great importance. Prior methods of accessing unique cells for storing data involved two write currents, each producing less magnetic stimulus required to write a cell, but together producing more magnetic stimulus than required to write the cell. However, due to many factors related to manufacturing uncertainties and intrinsic magnetic material variability, cell to cell magnetic response variations can be very large. This magnetic response variability from cell to cell directly impacts the electrical and resultant magnetic writing stimulus needed to access each cell for storing data, and therefore prevents array-wide selectivity to occur using a preferred, fixed electrical and resultant magnetic writing stimulus value.
The prior art approach to the magnetic cell variability problem has been to improve the manufacturing process. This has been challenging because the magnetic response of the cells is sensitive not only to local defects but also to edge and surface roughness.
What is needed is an MRAM array that is operable even when there is variability of magnetic properties among the cells.